Thèse de doctorat
Résumé : The Belgian nuclear research institute (SCK•CEN) is developing MYRRHA. MYRRHA is a flexible fastspectrum research reactor , conceived as an accelerator driven system (ADS). It is designed to operate in sub-critical and critical modes, with a proton accelerator producing the neutron source by spallation. Theconfiguration of the primary loop is pool-type: the primary coolant and all the primary system components (coreand heat exchangers) are contained within the reactor vessel, while the secondary fluid is circulating in the heatexchangers. The primary coolant is Lead Bismuth Eutectic (LBE). The recent nuclear disaster of Fukushima in2011 changed the requirements for the design of new reactors, which should include the possibility to removethe residual decay heat through passive primary and secondary systems, i.e. natural convection (NC).Verifying that sufficient mass flow takes place between the core and the heat exchanger in passive decay heatremoval conditions (core power=11MW) is necessary to fulfill the safety requirement. Analyzing the naturalconvection flow before building the reactor is therefore the primary objective of this work. Furthermore, it is ofinterest to understand which factors contribute to enhance or decrease the primary coolant mass flow. Inparticular, stratification and pressure drops in the primary hydraulic loop are expected to play important roles.The secondary objective of the study was therefore to verify the influence of the loop geometry and of theplenum on the mass flow.The methodology chosen to address the primary objective of the study consisted in measuring the quantities ofinterest in a 1:5 scaled water model (MYRRHABELLE) of the reactor. In particular the simultaneousmeasurement of mass flow, velocity field and temperature in natural convection working conditions is requiredto fulfill the first objective. Facilities of increasing size and complexity were used to develop experimentalmethodology and techniques before carrying out the experimental activities in MYRRHABELLE. It was find out thatenough mass flow occurs in the reactor's NC loop and that the upper plenum geometry influences flow andstratification